Dovecot not allowing logins to check email

A user reported that she was unable to get her email today and was receiving
an error in Outlook when she attempted to download her email. I logged into
the email server, which runs
Dovecot. I tried
establishing a telnet connection to the
POP3 port on
the server. I was able to successfully connect to the
well-known port,
TCP
port 110, used for POP3 connections. I could then enter the user's
username with the user command, but I never received a
reponse from the server, i.e. from Dovecot. I eventually had to hit
ctrl-], i.e., the Ctrl and "]" keys to return to the shell prompt.

I've encountered the same issue in the past, but haven't been able to
track down the cause yet. E.g., see
Dovecot not responding.
As I've done previously, I checked on entries in today's and yesterday's
mail logs for any entries that might be related to the problem and for
the number of dovecot processes running. I saw a lot of dovecot process
entries; 59 of those entries were pop3-login ones.

After checking on whether connectivity via IMAP was working, I checked
the number of pop3-login entries in today's mail log file again. I saw the
number had jumped from 59 to 73; I suspect now that those entries accumulate
as user's attempt to download email via POP3, but are unsuccessful.

# ps -aux | grep pop3-login | grep -c -v grep
73
#

As I've done previously, I restarted the dovecot service at this point,
so that users who use POP3 for their email will be able to download their
email.